Abstract

In plants, cellular innate immune responses are indispensable for defence against pathogens. Arabidopsis EDS1 (Enhanced Disease Susceptibility 1) and PAD4 (Phytoalexin Deficient 4) are essential regulators of basal resistance to virulent pathogens, controlling defence amplification and accumulation of the signalling molecule salicylic acid (SA). Also, EDS1 is necessary for Resistance (R) protein-triggered programmed cell death to avirulent pathogen isolates conditioned by the TIR (Toll-Interleukin-1 Receptor) class of nucleotide-binding/leucine-rich-repeat (NB-LRR) immune receptor. Complete loss of TIR-NB-LRR mediated resistance and its associated cell death programme in Arabidopsis eds1 mutants and partial disabling of the same resistances in pad4 suggested a mechanism in which TIR-type NB-LRR proteins engage EDS1 early in the defence cascade that connects the recognition process to basal defences, requiring both EDS1 and PAD4. Consistent with such a cooperative role, EDS1 and PAD4 interact in Arabidopsis soluble leaf extracts. EDS1 and PAD4 have homology to eukaryotic lipases in their N-terminal halves and share a domain of high sequence homology (the EP domain) in their C-termini with one other plant lipase-like protein, SAG101 (Senescence Associated Gene 101) that was recently identified as part of an EDS1 complex in leaf soluble extracts. However, the nature of this association and whether SAG101 signals in plant innate immunity was not known. The work presented here shows that SAG101 interacts directly with EDS1 inside the nucleus of Arabidopsis cells and, together with PAD4, contributes intrinsic and indispensable signalling activity to the EDS1 defence pathway in resistance and programmed cell death triggered by TIR-type R proteins and in expression of basal defences. The EDS1-SAG101 complex is molecularly and spatially distinct from EDS1-EDS1 homomeric interactions that occur in the cytosol but not in the nucleus. SAG101 possesses a defence regulatory function that is partially redundant with PAD4. Loss of SAG101 can be compensated for by the presence of PAD4. Single null sag101 mutant alleles had no effect on plant disease resistance but combining sag101 with a null pad4 mutation disabled resistance as fully as eds1. Restriction of SAG101 to the nucleus may account for its inability to fully complement loss of PAD4 that co-localises with EDS1 in the cytosol and the nucleus. These new findings demonstrate that all three proteins are important regulators of innate immunity and point to a complex nucleo-cytoplasmic dynamic between EDS1 and its signalling partners that may be important for plant defence signal relay.